This invention relates to electronic test and measurement instruments and, more particularly, to instrumentation for measuring characteristics of fiber optic systems, subsystems, and components. Specifically, the invention is directed to electronic measurement instruments for determining the continuity of elements in fiber optic systems, such as optical fiber cable.
Typical optical components of fiber optic systems include optical fiber cable and passive devices, such as switches, splitters, combiners, and attenuators. Various systems for measuring optical characteristics of these devices are known. The input and output signals for measurements on these devices are light, and the key parameters measured are attenuation versus modulation frequency, modulation bandwidth, optical delay, modal pulse dispersion, optical length, and distances to discontinuities. Discontinuities cause reflections in the optical component. Optical reflections and re-reflections produce standing wave patterns which create fading effects and crosstalk in optical communications systems.
Typically, discontinuities in fiber optic systems are located by a technique known as "optical-time-domain reflectometry" (OTDR). A measurement system which employs the OTDR technique is initially connected to the optical component to be tested. The next step of the OTDR technique is to generate a time domain optical pulse. This pulse is applied to the optical component being tested and is transmitted through the component until the terminus of the component is reached or until a discontinuity is encountered. A portion of the pulse is reflected at the termination or at any discontinuity in the optical component and propagates back to the measurement system. The measurement system then operates to sense the Raleigh back scatter to detect the reflected pulse energy.
Manufacturers of a few measurement systems which employ the OTDR technique claim to be able to resolve multiple reflections which are spaced at distances on the order of less than or equal to 5 cm. However, the measurement dynamic range of their equipment is limited. For example, although one manufacturer claims to resolve multiple reflections down to 1 mm spacing, the measurement dynamic range in terms of optical power is 30 dB.
Also, because the OTDR technique is employed to test long lengths of optical fiber cable and, also, because of inherent limitations on the amount of pulse energy which can be pumped into the cable, the measurement system typically has a high sensitivity so that remote discontinuities can be detected. Unfortunately, this results in "dead zones" where discontinuity measurements cannot be performed. These dead zones result when the pulse energy reflected from less remote discontinuities saturates the measurement system detector. Accordingly, it is desirable to avoid the dead zone problem inherent in the OTDR technique.
A known measurement system which avoids the limitations of measurement systems that employ the OTDR technique is shown in FIG. 1. Such a measurement system is disclosed, for example, in MacDonald, R. I., "Frequency domain optical reflectometers," Applied Optics, Vol. 20, No. 10, May 15, 1981, pages 1840-1844, and employs a Doppler frequency shift technique to detect one or more discontinuities in an optical fiber cable or to measure the length of the cable. The Doppler frequency shift measurement system disclosed in the MacDonald article comprises a fast swept modulation source connected through a power splitter to an electro-optical transducer to produce an intensity, or amplitude, modulated optical signal that is fed through an optical signal splitter/combiner to an optical fiber cable. The optical signal splitter/combiner produces a vectorial sum of the modulated optical signal incident on the optical fiber cable and any modulated optical signal reflected at a discontinuity or the terminus of the cable. The vectorial sum of the incident and reflected optical signals is converted to an electrical signal by an opto-electrical transducer. The electrical signal is then fed to a mixer where it is mixed with the modulation signal received from the power splitter, and the mixed product is fed to a spectrum analyzer which measures the frequency offset due to the travel time of the modulated reflected signal in the optical fiber cable. Unfortunately, the modulation frequency must be rapidly swept in order to generate a Doppler frequency shift. The rate at which such a modulation source can be swept, however, is limited by settling time of the circuit which produces the modulation signal.